Flowmeters for measuring the mass flow or flow rate through a pipe have included Coriolis flowmeters, magnetic flowmeters, ultrasonic flowmeters, vortex flowmeters and differential pressure flowmeters. When these flowmeters are to be adapted for very large diameter pipes such as pipes in excess of 14 inches, the size of the meters becomes excessively large or the flowmeters cannot be scaled up.
For instance, the largest pipe to be accommodated by a Coriolis type flowmeter is, for instance, a 12 inch diameter pipe. This flowmeter is however on the order of 4 to 6 feet high and is relatively massive, making it inconvenient at best.
There is in fact an urgent requirement for the ability to measure flow in large diameter pipes such as gas pipelines, oil pipelines, and the like and to be able to provide a flowmeter that is in-line insertable between or into these large diameter pipes.
It will be appreciated that the insertion of any blocking orifices or fluid blocking structures induces a downstream pressure drop which is largely unacceptable. For this reason it is only with difficulty that one can measure the mass or volumetric flow through a large diameter pipe to any degree of accuracy using traditional flowmeters.
More importantly it is important to be able to establish a measurement accuracy of 0.15% or better in order to accurately establish the mass or volumetric flow.
It will be appreciated that when oil, gas or other high value fluid is involved, measurement to 0.1% or 0.15% accuracy is important, especially in custody transfer applications when oil or gas is delivered from a wellhead to a remote location.
Typically Coriolis meters send all the fluid through an external tube or tubes and provide a flow measurement through the vibration of the tubes. These Coriolis devices have the required accuracy but, as mentioned before, when scaled up to a 12 inch diameter plus pipe, are exceedingly expensive and massive to the point of non-use. Note that when straight line Coriolis meters are used, the size of the pipe that can be accommodated is only of a size equal to or smaller than the straight line Coriolis tube, generally two inches.
Coriolis meters however do not exhibit excessive pressure drops and are therefore highly sought after.
Ultrasonic devices also produce minimal pressure drops and are highly reliable although they are somewhat less accurate than Coriolis flowmeters.
Magnetic flowmeters can only measure liquids or water, but cannot measure oil or gas', whereas vortex meters involving a bluff body in the fluid flow create vortices and provide a fluid flow measurement for oil as well. However, the vortex flowmeters involve pressure drops, with the bluff bodies often becoming dislodged with the presence of particles in the flow stream. Moreover, these meters are not particularly accurate and may be only accurate to 0.5 to 1%.
Differential pressure meters have been in existence for over 100 years and operate by creating an obstruction in the line to create a pressure drop. These meters then measure the difference in pressure upstream and downstream of the obstruction. In particular, many of these devices use an orifice plate as a primary element. However, the orifice plate can be dislodged or even installed backwards. The major problem with the differential pressure flowmeters is that they operate by creating a pressure drop. Note that for differential pressure flowmeters to work the orifice needs to be relatively small; but when inserted into a large diameter pipe, the pressure drop is unacceptable.
There are of course turbine meters that have spinning rotors or paddles to measure volumetric flow in terms of the rotation of the spinning wheel. However, these spinning wheels have ball bearings that wear out and the presence of the turbine causes a significant pressure drop. Moreover, scaling these meters up to accommodate large diameter pipes is a problem due to the mass of the parts involved.
Presently there are the relatively new sonar flowmeters that instead of creating vortices, measure natural swirls in a stream to create a flow measure. However these sonar devices have limited success in the pulp and paper industry in which dirt and other contaminants are in the flow, as well as being relatively unsuitable for dirty water applications such as the waste water measurements required in metals or mining operations.
Finally the last type of flowmeter is the photo-optical flowmeter, which injects a beam of light into the flow.
The problems with all of the above flowmeters, aside from the ones described above, is that a large diameter pipe must be necked down to a convenient size in order to measure flow.
The necking down of these large diameter pipes in itself produces pressure drops that must be accounted for downstream, usually by adding additional pumping stations.
There is therefore a requirement to be able to provide an in-line flowmeter for large diameter pipes from 10 inches to 48 inches, and to be able to accurately measure the mass or volumetric flow in the pipe without creating downstream pressure losses.
It is noted that while Coriolis flow measuring devices are thought to be of the highest accuracy in the field, their biggest drawback is pipe size, where presently 70% of the coriolis meters are sold for pipe sizes of 2 inches and under.
When one tries to extend the capability of such Coriolis meters to pipe sizes for instance above 14 inches, it is indeed with only with great difficulty that one can create the proper vibrations within the Coriolis tube or tubes when one needs to size up these tubes. Moreover for larger Coriolis flowmeters on the order of 4 to 6 feet in height, they are so heavy that they require specialized shipping crates to accommodate weights in hundreds of pounds due to the massive amount of steel that is utilized. Thus, the majority of Coriolis flowmeters can only accommodate pipes of 6 inches or less in diameter while still maintaining the 0.1% accuracies.
What is therefore needed is a technology that can utilize the advantageous parts of any flowmeter and yet permit their use for large diameter pipelines, namely for pipes exceeding 14 inches.